Single and multi-stage high power optical isolators using a single polarizing element

ABSTRACT

An optical isolator for generally collimated laser radiation includes a single polarizing element, at least one Faraday optical element, at least one reciprocal polarization altering optical element disposed at the single polarizing element, at least one reflective optical element for reflecting radiation to provide an even number of passes through the at least one Faraday optical element, and a magnetic structure. The magnetic structure is capable of generating a magnetic field within the at least one Faraday optical element that is generally aligned with the even number of passes along a beam propagation axis. The optical isolator is configured to receive generally collimated laser radiation, which passes through the single polarizing element and the at least one reciprocal polarization altering optical element and which makes at least two passes through the at least one Faraday optical element, whereby generally collimated laser radiation is output from the optical isolator.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the filing benefits of U.S. provisionalapplication Ser. No. 62/269,349, filed Dec. 18, 2015, which is herebyincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to high power optical isolators and moreparticularly to high power single and multi-stage polarizationmaintaining [“PM”] and polarization insensitive [“PI”] opticalisolators.

BACKGROUND OF THE INVENTION

Optical isolators are routinely used to decouple a laser oscillator fromdownstream laser amplifier noise radiation and/or target reflections.Optical isolators are typically comprised of a Faraday rotatorsurrounded by polarizers that are aligned with the input and outputlinear polarization states. A Faraday rotator is typically comprised ofa non-reciprocal, optical element in a strong magnetic field that isco-axially aligned with the laser radiation so that the plane ofpolarization is rotated by 45 degrees. In an optical isolator, thenon-reciprocal nature of the Faraday effect causes the plane of linearpolarization in the backward propagating direction to be rotated anadditional 45 degrees resulting in a polarization state which is 90degrees to the transmission axis of the input polarizer. This results inreverse propagating radiation to experience high transmission losseswhile allowing forward propagating radiation to experience lowtransmission losses. Optical isolators suitable for randomly polarizedlight are also common and are termed polarization insensitive [“PI”]isolators such as disclosed in U.S. Pat. No. 4,178,073.

The sensitivity of distributed feedback diode lasers and othercomponents in telecom systems to feedback from backward propagatingradiation has prompted the development of multi-stage PI isolators toincrease isolation to 60 dB. An example of such a multi-stage isolatoris a two stage device disclosed in U.S. Pat. No. 5,237,445. To addressthe bulk and expense of high power Faraday rotators in the nearinfrared, the amount of Faraday rotation as given by the followingequation has been examined:

θ(λ,T)=V(λ,T)×H(r,T)×L _(F)   (1)

where:

-   -   λ(λ,T): The Faraday rotation angle (a function of wavelength, λ,        and temperature, T);    -   V (λ,T): A proportionality constant, termed the Verdet constant,        of the Faraday element (a function of wavelength, λ, and        temperature, T);

H(r,T): The strength of the magnetic field in the direction of lightthrough the Faraday element (a function of radial position r across thebeam and temperature, T); and

-   -   L_(F): The optical path length within the Faraday element.

Equation 1 states that the Faraday rotation angle can be increased byeither an increase in the Verdet constant V (λ,T), the magnetic fieldstrength H(T), or the Faraday element length L_(F). In order to make anoptical isolator as small and inexpensive as possible, multi-passFaraday rotators have been disclosed, such as in U.S. Pat. Nos.4,909,612; 5,715,080 and 7,057,791, which are hereby incorporated byreference in their entireties. The reduced gap between the magnetssignificantly improves the magnetic efficiency and uniformity of themagnetic assembly. As disclosed in U.S. Publication No. 2015/0124318(which is hereby incorporated herein by reference in its entirety),these improvements increase the effective magnetic field H(r,T) andallow for reductions in the optical path length L_(F), magnetic materialvolume, and Faraday optic volume. The reduction of the optical pathlength L_(F), is additionally advantageous in high power applicationsfor reductions in beam degradation due to absorption.

A fiber to fiber optical isolator for low power laser radiation isdisclosed in U.S. Pat. No. 5,499,132. This optical isolator isapplicable to only fiber to fiber devices and requires an internalfocusing lens for proper operation. In addition, optical damage due tohigh fluence levels of the small beam diameters at the fibers as well asin the birefringent crystal plate prevents scaling of this device topowers above 10W.

SUMMARY OF THE INVENTION

The present invention provides PM and PI multi-pass isolator forms thatare simple to align, have few optical parts to assemble and work wellwith scalable beam diameters large enough to prevent optical damage inthe optical elements of the isolator when used with high peak andaverage power laser sources. The present invention relates to high peakand average power optical isolators and more particularly to high powersingle and multi-stage polarization maintaining [“PM”] and polarizationinsensitive [“PI”] optical isolators with improved size, improvedalignment simplicity, reduced parts count, and reduced cost.

According to an aspect of the present invention, an optical isolator forgenerally collimated laser radiation is provisioned with one or moreisolation stages using a single polarizing element, a multi-pass Faradayrotator through which light passes an even number of times and one 45degree reciprocal polarization rotation element per isolation stage. Thesingle polarizing element enables simple alignment and reduced partscount in an optical isolator that is scalable in beam diameter for highpower operation.

In a preferred embodiment, the multi-pass Faraday rotator comprises aFaraday optic with a highly reflective coating on one optical face andan anti-reflective coating on the opposite optical face nearest to thesingle polarizing element. A magnetic field generally aligned to thebeam path in the multi-pass Faraday rotator causes 45 degreenon-reciprocal polarization rotation in the Faraday optic for eachisolation stage.

The 45 degree reciprocal polarization rotation element may comprise aquartz wave plate and may be bonded, such as by adhesive free opticalcontact for high power applications, to the surface of the singlepolarizing element at only a portion of the single polarizing elementsuch that the wave plate is located in only one pass of the beam pathand is aligned for the opposite sense rotation that is opposite to theFaraday non-reciprocal rotation in the transmission direction.

Optionally, a high reflection region may be coated onto the singlepolarizing element's optical face that is adjacent to theanti-reflection coated surface of the Faraday optic, whereby increasedbeam overlap and reduced overall size can be realized for an even numberof passes greater than two. If the single polarizing element is a fusedsilica polarizing beam splitter, a polarization maintaining or PMisolator suitable for high power with only two separate opticalcomponents is realized. Similarly, if the single polarizer element is afused silica polarization splitting beam displacer, a polarizationinsensitive or PI isolator with only two separate optical components isrealized with no critical alignment required. Both forms of isolatorshave readily scalable beam diameters for high power operation. Highpower beam quality is limited only by the thermal optic properties ofthe Faraday rotation optical element.

The present invention thus is an improvement over the systems anddevices described in U.S. Pat. Nos. 4,909,612; 5,715,080 and 7,057,791,where two or more polarizing elements are used and high reflection facesare placed upon opposing optical faces of multi-pass Faraday optics topromote more than two passes through the Faraday optic. In contrast tothe systems disclosed in these patents, the system of the presentinvention uses a single polarizing element to simplify constructionwhile also reducing size and cost. In the multi-stage isolator form, thesystem of the present invention can be constructed with only two opticalcomponents and has the further benefit that the isolator isself-aligning with respect to polarization throughout the multi-stageisolator. In addition, the use of high reflection coating region(s) onthe optical face of the polarizing element closest to the Faraday opticimproves the magnetic efficiency by increasing the number of beam passesthrough the Faraday optic while maintaining beams that are generallyparallel to the magnetic field, thereby reducing the thickness of theFaraday optic and minimizing the required magnetic structure. Thesedistinctions can all be made while increasing the beam size, asrequired, to prevent optical damage to optical elements within theisolator to scale the power as desired up to the limit imposed by thethermal optic properties of the Faraday optic material used.

In accordance with another aspect of the present invention, multi-stageisolators can be realized by adding a high reflection coating region onthe single polarizing element's optical surface furthest from theFaraday optic between duplicate isolation stages such as described abovefor the preferred embodiment. In the case of a PI isolator, this highreflection coating is best applied to the non-bonded external surface ofa quartz quarter-wave plate to flip polarizations between pairs ofisolation stages to make path-lengths identical for both polarizations.The quarter wave-plate may be first bonded, such as by optical contact,with its optic axis aligned 45 degrees to each polarization axis of thefused silica polarization splitting beam displacer. Again, simplemultiple stage isolators with only two separate optical components thatare self-aligning with respect to polarization for easy assembly arepossible.

These and other objects, advantages, purposes and features of thepresent invention will become apparent upon review of the followingspecification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a PM single stage, dual pass isolator(showing forward propagation of the beam) in accordance with the presentinvention;

FIG. 1B is another perspective view of the PM single stage, dual passisolator of FIG. 1A, showing reverse propagation of the beam;

FIG. 1C is a perspective view of a PM single stage, quad pass isolatorin accordance with the present invention;

FIG. 1D is a perspective view of a PM dual stage, dual pass isolator inaccordance with the present invention;

FIG. 2A is a perspective view of a PI single stage, dual pass isolator(showing forward propagation of the beam) in accordance with the presentinvention;

FIG. 2B is another perspective view of the PI single stage, dual passisolator of FIG. 2A, showing reverse propagation of the beam;

FIG. 3A is a perspective view of a PI dual stage, dual pass isolator(showing forward propagation of the beam) in accordance with the presentinvention;

FIG. 3B is another perspective view of the PI dual stage, dual passisolator of FIG. 3A, showing reverse propagation of the beam; and

FIG. 4 is a perspective view of a PI dual stage, dual pass isolator(showing reverse propagation of the beam) with a triple layer displacerin accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and the illustrative embodiments depictedtherein, FIG. 1A is a perspective view of a PM single stage, dual passisolator for generally collimated laser radiation that is comprised ofonly two separate optical components in accordance with the invention.Collimated laser radiation source 1100 with substantially linear ppolarization is incident upon single polarization element 1101 comprisedof quartz half-waveplate 1103 bonded, such as by adhesive free opticalcontact, to fused silica PBS 1102. The optic axis of half-waveplate 1103is rotated by an angle of 22.5 degrees with respect to the originallinear p polarization axis (shown as vertical in FIG. 1A), such thathighly polarized p polarization transmitted through polarizing coating1108 is rotated by +45 degrees of reciprocal rotation in half-waveplate1103 as shown by the circled polarization state arrow 1109.

Laser radiation at 1109 is then incident upon Faraday optic 1104 (suchas a Terbium Gallium Garnet (TGG) or other suitable Faraday opticelement) which is immersed in a magnetic field that is generally alignedto the beam path, where the beam receives −22.5 degrees ofnon-reciprocal Faraday polarization rotation for each pass of Faradayoptic 1104. Reflection coating 1105 on Faraday optic 1104 facilitatesthe dual pass beam path in Faraday optic 1104 for −45 degrees of totalnon-reciprocal polarization rotation which restores the original ppolarization state in beam 1106′ which is then re-incident upon singlepolarization element 1101 at point 1110 (where the re-incident locationis at a location that does not have the half wave plate 1103 at thepolarization element). Radiation from point 1110 is highly p polarizedoutput radiation 1106 upon final pass through the single polarizationelement 1101. In view of the angle β between collimated input radiation1100 and output radiation 1106 as well as the separation L1 betweensingle polarization element 1101 and Faraday optic 1104, half-waveplate1103 is dimensioned and positioned to not clip input 1100 or output 1106radiation.

The single stage, dual pass isolator of FIG. 1A also limits or precludeslight traveling in the reverse direction from reaching the source oflight beam 1100 in FIG. 1A. As shown in FIG. 1B, light 1120 received atsingle polarizing element 1101 at the side or region of the element thatdoes not have the half wave plate 1103, passes through the singlepolarizing element and through the faraday optic 1104, which, after itreflects off reflector 1105 so as to make a second pass through theoptic 1104, is rotated −45 degrees (beam 1121), which then passesthrough the half wave plate 1103, which rotates the beam −45 degrees,such that the beam 1122 is an s polarized beam 1122. Such an s polarizedbeam is reflected by the polarizing coating 1108 away from the source ofthe light beam 1100.

Optionally, if more than two passes are desired through the PM singlestage isolator, a reflector may be added at the single polarizingelement such that light (after two passes through the faraday optic)reflects back toward the faraday optic for a third and fourth pass (sucha single stage, quad pass isolator may be suitable for small magnets ora low Verdet constant faraday optic). Such a configuration is shown inFIG. 1C, where the single polarizing element 1201 has a half wave plate1203 and a polarizing coating 1208 similar to single polarizing element1101, discussed above, and the isolator includes a Faraday optic 1204and reflector 1205 similar to the optic 1104 and reflector 1105discussed above. Single polarizing element 1201 includes a reflector1211, such as at a center region between the half wave plate 1203 andthe point or region 1210 where the light beam passes back through thesingle polarizing element 1201. In this configuration, the Faraday optic1204 comprises a thinner optic or less powered magnetic field, such thateach pass through the optic only rotates the polarized light 11.25degrees (half the rotation achieved by Faraday optic 1104, discussedabove), whereby after four passes through the Faraday optic 1204, thelight is rotated +45 degrees to counter the −45 degrees rotationachieved by the half wave plate 1203. Thus, the light output from theFaraday optic (after the fourth pass) is “p” polarized light.Optionally, single polarizing element 1201, Faraday optic 1204, andreflector 1211 could be increased in size to support additional passesof the Faraday optic 1204; further reducing the thickness of the Faradayoptic or size of the magnetic structure (such as the magnetic structure1215 of FIG. 1C, which may have two separate magnetic elements 1215 a,1215 b, and/or which may be adjustable, as discussed below).

Optionally, a dual stage, dual pass isolator may include two half waveplates, one for the light at an input region of the single isolator andanother at an output region of the single isolator. For example, andsuch as shown in FIG. 1D, a single polarizing element 1301 has a halfwave plate 1303 a and a polarizing coating 1308 similar to singlepolarizing element 1101, discussed above, and the isolator includes aFaraday optic 1304 and reflector 1305 similar to the optic 1104 andreflector 1105 discussed above. Single polarizing element 1301 furtherincludes a reflector 1311, such as at a center region between the halfwave plate 1303 a and the point or region 1310 where the light beampasses back through the single polarizing element 1301. The singlepolarizing element 1301 also includes a second half wave plate 1303 b atregion 1310, such that the light from the Faraday optic 1304 (after itsfourth pass through the Faraday optic) is rotated as it passes throughthe second half wave plate 1303 b so that the exiting beam is a ppolarized beam. This is because a p polarized input beam will rotate −45degrees when passing through first half wave plate 1303 a, and then willrotate back +45 degrees when making two passes through the Faraday optic1304, and then will rotate an additional +45 degrees when making anothertwo passes through the Faraday optic (after reflecting off of reflector1311). The beam then is rotated back −45 degrees by the second half waveplate 1303 b to be a p polarized beam as it exits the single polarizingelement 1301.

The isolator of the present invention thus provides multiple passesthrough a Faraday optic. If only two passes are made through the Faradayoptic, then more magnetic power may be needed at the optic, which mayresult in a larger package. By providing for four or more (even numberof) passes through the Faraday optic, a smaller magnet package may beused at the Faraday optic.

Referring to FIG. 2A, a perspective view of a single stage, dual pass PIisolator 1400 is shown in accordance with the present invention. Theisolator includes a polarizing element 1401 and a Faraday optic 1404,with a half wave plate 1403 disposed at part of the polarizing elementand a reflector 1405 disposed at the Faraday optic 1404. In theillustrated embodiment, the polarizing element 1401 has a firstpolarizing coating 1414 and a second polarizing coating 1415 at thediagonal surfaces of parallelepiped 1412 that are similarly bonded, suchas through optical contact, to fused silica prisms 1413. Randomlypolarized radiation 1410 from a source in the forward direction isresolved into “p” and “s” polarized beams 1416 and 1417, respectively,at the first polarizing coating 1414, with the s polarized beam beingreflected upward by polarizing coating 1414. The “s” polarized beam 1417is further reflected at the second polarizing coating 1415 and thentransmitted out of the output AR coated surface of parallelepiped 1412precisely parallel to and displaced from the “p” polarized beam 1416. Asshown in FIG. 2A, the beams pass through the wave plate 1403 where theyare rotated −45 degrees, and then the rotated p and s beams pass throughthe Faraday optic 1404 and reflect off of the reflector 1405, so as tomake two passes through the optic 1404 (where they are rotated back +45degrees) and return towards the polarizing element 1401. The beams enterthe polarizing element at a location devoid of the half wave plate 1403,whereby the s beam is reflected downward by the polarizing coating 1415and further reflected by the polarizing coating 1414 so as to exit thepolarizing element 1401 with the p beam.

For light traveling in the reverse direction (FIG. 2B), the 90 degreerotation (+45 degrees by the Faraday +45 degrees by the wave plate) inthe reverse direction causes the upper beam (the beam reflected by thepolarizing coatings 1414, 1415) to pass through the upper diagonal (notreflected by the polarizing coating 1415) and the lower beam to bereflected by the lower diagonal polarizing coating 1414. Thus, reversepropagation of light is precluded from exiting the isolator in thedirection of the source of light beam 1410 of FIG. 2A. Therefore,reverse propagating light is unable to couple back into the source.

A two stage PI isolator is increasingly desired to manage backreflections from PI laser systems generating over 100W of average power.Leakage from traditional single stage isolators can be sufficient to beamplified to levels which are harmful to internal laser components.Referring to FIG. 3A, a perspective view of a dual stage, dual pass PIisolator 1500 is shown in accordance with the present invention. Theisolator includes a polarizing element 1501 with half wave plates 1503 aand 1503 b and quarter wave plate 1511 disposed at part of thepolarizing element and a reflector 1505 disposed at the Faraday optic1504. Collimated laser radiation 1510 is incident upon a single fusedsilica polarizing beam displacer 1501 (PBD) where it is resolved intoforward propagating “s” polarization beam 1516 (solid line) and “p”polarization beam 1517 (dashed line). Both “p” and “s” polarizationbeams are transmitted through first isolation stage quartz halfwaveplate 1503 a which may be bonded to beam displacer 1501 with opticaxis +22.5 degrees such that the “p” and “s” polarized beams experiencea +45 degree polarization rotation about the forward propagation axis tobe +45 and +135 degree polarized beams, respectively.

The beams 1516, 1517 then make two passes through the Faraday optic 1504(via reflection off of reflector 1505) and return to the PBD 1501, wherethe p beam passes through the PBD and through a ¼ wave plate 1511 andreflects off a reflector at the back side of the wave plate 1511 so asto again propagate through the PBD 1501. The s beam also passes throughthe PBD and reflects off of the upper reflector coating and again off ofthe lower reflector coating so as to pass through the ¼ wave plate 1511and reflects off a reflector at the back side of the wave plate 1511 soas to again propagate through the PBD 1501.

After the first pass of the ¼ wave plate 1511, the light is circularlypolarized. The reflection off of the backside of the ¼ wave plate causesa 180 degree phase shift thereby reversing the circularity. The returnpass through the ¼ wave plate converts the light back to being planarpolarized, but with the light then being rotated 90 degrees, such thatthe s beam becomes a p beam and the p beam becomes an s beam. Thisallows the two beams to flip planes and travel the same path length. Inother words, the now s beam 1517′ (formerly the p beam) now reflects offof the coatings in the PBD, while the now p beam 1516′ (formerly the sbeam) now passes directly through the PBD. Thus, by the time the twobeams have again passed through the Faraday optic and again passedthrough the PBD so as to exit the PBD as beam 1510′, the s beams and pbeams have traveled the same path length. For collimated laser light,this is very important and allows very high beam quality to bemaintained. If a particular application does not require high beamquality, ¼ wave plate 1511 could be removed and replaced with a highreflection coated region.

Thus, and as shown in FIG. 3A, the two beams will be reflected betweenFaraday optic high reflector 1505 (at the rear or opposite end of theFaraday optic 1504) and single fused silica PBD first stage highreflector 1511 N times such that 2N passes are made through Faradayoptic 1504 before the two beams are incident upon single fused silica ARcoated region of the beam displacer 1501.

Reverse propagating radiation 1520 (FIG. 3B), will follow the identicalray path and polarization states until after the rays have beentransmitted through the second stage half-waveplate where the −45degrees of second stage Faraday rotation is added to −45 degrees ofreciprocal second stage half-waveplate rotation to rotate thepolarizations of both beams by −90 degrees such that they are rejectedaway from the forward beam propagation axis as shown by rejected raylines in FIG. 3B. As shown in FIG. 3B, one beam of reverse propagatingbeam 1520 passes through the PBD 1501 and wave plate 1503 b and reflectsback from the Faraday optic 1504, whereby it is rotated such that itreflects downward at the lower reflector coating of the PBD 1501 andexits as a first stage rejected beam 1521 a. The other polarized beam ofthe initial reverse propagating beam 1520 reflects upward in the PBD1501 and further reflects to the Faraday optic and back, where it isrotated such that it passes through the PBD 1501 and exits the PBD asfirst stage rejected beam 1521 b.

After the residual radiation is reduced in power by typically 30 dBrelative to the original back-reflected power in the second stageisolator, the polarizations are again flipped by quarter waveplatebefore repeating the process in the first isolation stage, where again−45 degrees of first stage Faraday rotation is added to −45 degrees ofreciprocal first stage half-waveplate rotation to again rotate thepolarizations of both beams by −90 degrees such that they are once againrejected away from the original forward beam propagation axis as shownby the lines 1522 a, 1522 b in FIG. 3B. With the first stage 30 dBisolation similar to the second stage 30 dB isolation, the residualradiation from the original back-reflected power level is reduced byapproximately 60 dB, thereby ensuring that none of this back-reflectedlight can damage or disrupt the laser system.

Referring now to FIG. 4, an isolator 1600 comprises a single polarizedbeam displacer 1601 having three reflective layers or coatings 1614 a,1614 b, 1614 c. The isolator 1600 operates in a similar manner toisolator 1500, discussed above, for forward propagating beams. However,by adding a third layer to the displacer, all four rejected beams 1621a, 1621 b, 1622 a, 1622 b can be directed in the same direction as theyexit the isolator. This simplifies the beam dump and thermal managementof the high power reverse power.

The reciprocal polarization rotators need not be half-waveplates, theycould also be (quartz) optical rotators, for example, or other suitablereciprocal polarization rotators. All of the above quartz waveplatesneed not be bonded to the single fused silica PBD, however aligningtheir optical axis and then bonding such waveplates to fused silicaoptical components such as polarizing beam-splitter cubes by opticalcontact is desired. Bonding these quartz waveplates directly to thesingle fused silica PBD to form a single optical part during finalassembly has the advantage of greatly reducing the overall cost, partscount and assembly time for the optical isolator of this invention.Thus, the present invention provides a high performance PI isolator thatis scalable in power with beam diameter that can be fabricated with onlytwo separate optical components.

Although specifics above were given for a TGG Faraday optic, any Faradayoptic material may be used in accordance with the present invention,such as, for example, ferromagnetic, paramagnetic, semiconductor anddiamagnetic materials and/or the like. In particular, diamagneticmaterials which typically have a very low Verdet constant but often haveextremely low absorption can function well as temperature insensitiveoptical isolators in accordance with the invention because their Verdetconstant is only very weakly related to temperature. The specific signsof reciprocal and non-reciprocal rotation need not be limited to thosedescribed above—they can be mutually reversed by reversing the sign ofthe applied magnetic field to the Faraday optic and rotating thedirection of the reciprocal polarization rotators accordingly.

High reflection coatings should impart a pure 180 degree phase shiftupon reflection and need not be limited to thin films as they can alsobe made from metal coatings.

The Faraday optics high reflection coated surface may have a protectiveoverlayer, such as SiO₂ or the like, and then a metallization layer,such as gold or the like, so that the Faraday optic may be soldereddirectly to a heat sinking housing with, for example, a gold-tin solderlayer for enhanced conduction of heat out through the high reflectioncoated surface. Heat flow substantially parallel to the beam pathminimizes any radial heat flow across the beam cross section that canresult in thermal lens focal shifts and thermal birefringence.

Optionally, it is another aspect of the present invention that theFaraday optic may comprise a layered structure with a transparent heatconductive layer bonded to one or both optical faces of a diamagnetic,paramagnetic or ferromagnetic Faraday rotating material. Suchtransparent heat conductive layers, in conjunction with sufficientmulti-passes to ensure that the Faraday optic is thin relative to thebeam diameter, ensures that heat flow is substantially parallel to thebeam path within the Faraday optic. The function of the transparent heatconductive layer is described in detail in U.S. Publication No.2014/0218795, which is hereby incorporated herein by reference in itsentirety. Heat flow parallel to the beam path eliminates radialtemperature gradients responsible for thermal lens focal shift andthermal birefringence.

Another aspect of the present invention is that the multi-pass Faradayrotator may use an adjustable magnetic structure that is capable ofmodifying the magnetic field strength generally aligned to the beam pathwith the Faraday optical element(s) used in the multi-pass Faradayrotator. In the case of multi-stage optical isolators, such magneticfield adjustability can be independent or different for each stage forimproving the temperature and/or wavelength bandwidth performance of theoptical isolator. The adjustable magnetic structure is adjustablerelative to the optical elements via any suitable electrical ormechanical or electromechanical means that may adjust the space or gapbetween the magnetic structure and the optical element to provide thedesired performance of the optical isolator. For example, and such asshown in FIG. 1C, an adjustable magnetic structure 1215 may include twomagnetic structures 1215 a and 1215 b, which can be moved independentlyor in tandem (relative to the Faraday optic 1204) to achieve differentfaraday rotations per stage.

Therefore, the present invention provides an optical isolator having oneor more isolation stages using a single polarizing element inconjunction with a multi-pass Faraday rotator and one 45 degreereciprocal polarization rotation element per isolation stage forimproved alignment simplicity, reduced parts count and lower cost. Themulti-pass Faraday rotator optionally and desirably has an even numberof multi-passes and may comprise a Faraday optic with a highlyreflective coating on one optical face and an anti-reflective coating onthe opposite optical face nearest to the single polarizing element. Amagnetic field generally aligned to the beam path in the multi-passFaraday rotator causes 45 degree non-reciprocal polarization rotation inthe Faraday optic for each isolation stage. The 45 degree reciprocalpolarization rotation element may comprise a quartz waveplate that isbonded, such as by adhesive free optical contact for high powerapplications, to a surface of the single polarizing element in theoptical path of only one pass of the beam and aligned for the oppositesense rotation to the Faraday non-reciprocal rotation.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the principles of the invention,which is intended to be limited only by the scope of the appendedclaims, as interpreted according to the principles of patent lawincluding the doctrine of equivalents.

1. An optical isolator for generally collimated laser radiation with oneor more isolation stages, said optical isolator comprising: a singlepolarizing element; at least one Faraday optical element comprisingopposing optical faces through which there is a beam propagation axis;at least one reciprocal polarization altering optical element disposedat said single polarizing element; at least one reflective opticalelement for reflecting radiation to provide an even number of passesthrough said at least one Faraday optical element; a magnetic structurecapable of generating a magnetic field within said at least one Faradayoptical element that is generally aligned with the even number of passesalong said beam propagation axis; and wherein said optical isolator isconfigured to receive generally collimated laser radiation, which passesthrough said single polarizing element and said at least one reciprocalpolarization altering optical element and which makes at least twopasses through said at least one Faraday optical element, wherebygenerally collimated laser radiation is output from said opticalisolator.
 2. The optical isolator of claim 1, wherein said at least onereflective optical element comprises a reflective coating at a surfaceof said at least one Faraday optical element.
 3. The optical isolator ofclaim 1, wherein said at least one reflective optical element comprisesa reflective coating at a surface of said polarization altering opticalelements.
 4. The optical isolator of claim 1, wherein said at least onereflective optical element comprises a reflective coating at a surfaceof said single polarizing element.
 5. The optical isolator of claim 1,wherein said at least one reflective optical element comprises a mirror.6. The optical isolator of claim 1, wherein said at least one reciprocalpolarization altering optical element comprises at least one half-waveplate.
 7. The optical isolator of claim 1, wherein said at least onereciprocal polarization altering optical element comprises at least onequarter-wave plate.
 8. The optical isolator of claim 1, wherein said atleast one reciprocal polarization altering optical element comprises atleast one optical rotator.
 9. The optical isolator of claim 1, whereinsaid single polarizing element comprises a polarizing beam displacer.10. The optical isolator of claim 9, wherein said polarizing beamdisplacer comprises a birefringent crystal.
 11. The optical isolator ofclaim 9, wherein said polarizing beam displacer comprises a beamdisplacer configured so that all backward propagating radiation isdirected in the same direction as it exits the optical isolator.
 12. Theoptical isolator of claim 1, wherein said magnetic structure comprisesan adjustable magnetic structure capable of modifying the magnetic fieldstrength within said at least one Faraday optical element.
 13. Theoptical isolator of claim 1, wherein said magnetic structure comprisesan adjustable magnetic structure capable of modifying the magnetic fieldstrength within each isolation stage independently.
 14. The opticalisolator of claim 1, wherein said at least one Faraday optic elementcomprises a layered structure with a transparent heat conductive layerbonded to one or both optical faces of a diamagnetic, paramagnetic orferromagnetic Faraday rotating material.
 15. The optical isolator ofclaim 1, wherein reverse propagating radiation passes through saidsingle polarizing element and said at least one reciprocal polarizationaltering optical element and makes at least two passes through said atleast one Faraday optical element, whereby the reverse propagatingradiation is not output from said single polarizing element in thedirection of the source of the generally collimated laser radiation.